With the advent of micromachining techniques, there has been renewed interest in electrostatic actuators sometimes called "micromotors". Electrostatic actuators achieve high energy densities and can be manufactured using straightforward manufacturing techniques. Electrostatic actuators have been used to position optical devices, to operate switches, and to turn small gears. For advanced data storage devices and other applications, micromachined actuators that have a large travel, whose positioning can be controlled with great precision, and that operate in response to a low actuation voltage are needed. These requirements are not met by known micromachined electrostatic actuators.
A micromachined electrostatic actuator that satisfies some of the above requirements is described by Trimmer and Gabriel in Design Considerations for a Practical Electrostatic Micro-Motor, SENSORS AND ACTUATORS, Vol. 11, pages 189-206 (1987) and in U.S. Pat. No. 4,754,185. These documents describe an electrostatic actuator in which a grounded moveable silicon substrate or "rotor" (sometimes called a "translator") is moved relative to a fixed silicon substrate or "stator." The stator has several sets of electrodes on its surface, one of which is held at a voltage different from ground in order to position the rotor. Stepped motion is provided by setting the pitches of the stator and rotor electrodes in a vernier relationship. The rotor electrodes all having the same voltage, i.e., ground potential, significantly eases fabrication of the device.
However, the electrostatic actuator described by Timmer and Gabriel does not meet all of the requirements set forth above. For example, an actuation voltage of approximately 100 V is required to exert a force on the rotor in the direction parallel to the plane of the rotor surface (an "in-plane force") in the range of forces required to operate an advanced memory device. This actuation voltage is outside the range of voltages that can be controlled using CMOS integrated circuits. Moreover, the in-plane force is accompanied by an out-of-plane force perpendicular to the plane of the rotor. The out-of-plane force attracts the rotor towards the stator and is as much as ten times greater than the in-plane force.
The large attractive out-of-plane force places significant constraints on the suspension used to maintain the spacing between the rotor and stator. For conventional-size electrostatic actuators, spacers, bearings and lubricating layers may be used to support the rotor against the attractive force. However, for micro-scale structures, it is more difficult to provide an effective way of maintaining the spacing between the rotor and stator without large frictional forces that adversely affect operation.
Folded beam flexures are most commonly used in micromachined devices to support the rotor above the stator. Advanced data storage applications require actuators that can travel 25 .mu.m laterally while maintaining the rotor-stator spacing to an accuracy of 0.1 .mu.m. If the ratio of the out-of-plane force to the in-plane force is near 10, as in the electrostatic actuator described by Timmer and Gabriel, then a 2 .mu.m-wide beam flexure would need to be at least 100 .mu.m tall to have sufficient out-of-plane stiffness. Such a structure is extremely difficult to fabricate using conventional processing.
A first approach to mitigate the effects of the out-of-plane attractive forces in micromachined devices is to use two stationary electrode plates on opposite sides of a movable plate. By selecting the appropriate electrode configuration, it is possible to levitate the moving plate at a relatively stable position between the two stationary plates. However, this approach requires exacting process control during fabrication and/or assembly.
A second known approach applicable to micromachined devices is to use the weight of the movable substrate to counteract the attractive force. However, since this approach does not work if the electrostatic actuator is tilted, its usefulness is significantly restricted.
In both of the approaches discussed above, the rotor electrodes are all held at a single voltage. Macro-scale electrostatic actuators are known that have three or more voltages present on both the stator and rotor. One approach using a three-phase oscillating voltage pattern is described in U.S. Pat. No. 5,534,740 of Higuchi et al. This approach can produce a very large in-plane force. However, the large in-plane force is accompanied by a large out-of-plane force about four times greater than the in-plane force. Oscillating voltages of approximately 200 volts are required to generate an in-plane force of sufficient magnitude to overcome friction in the suspension elements. Therefore, this approach will not conveniently scale to a micromachined device because of the large out-of-plane force and the requirement to connect three oscillating voltages to the rotor electrodes. Making electrical connections to a moveable rotor is difficult, particularly for a micromachined rotor, so it is desirable to minimize the number of voltages present on the rotor electrodes. In addition, the way in which the voltages vary with time should be made as simple as possible.
Some conventional electrostatic actuators provide precise position control and a large range of travel, but cannot simply be scaled for use in micromachined electrostatic actuators. This is because these actuators operate with actuation voltages greater than those that can be controlled using CMOS integrated circuits, generate an out-of-plane force that is too large relative to the in-plane force, and require too many electrical connections to be made to the rotor.
What is needed is an electrostatic actuator and a way of controlling an electrostatic actuator that provides precise positioning and that can be controlled using CMOS integrated circuits. What is also needed is such an electrostatic actuator that can be fabricated using micromachining techniques that employ processing similar to that used to make integrated circuits.
U.S. patent application 08/818,209, filed Mar. 14, 1997, entitled "Electrostatic Actuator With Alternating Voltage Patterns", having inventors Storrs Hoen and Carl Taussig, (hereafter referred to as the "previous invention") which is incorporated by reference (but is unpublished at the filing of the instant patent application), describes an electrostatic actuator, hereinbefore unknown in the art, that partially satisfies these needs. An alternating voltage pattern is imposed on electrodes located on opposed electrode surfaces of both the rotor and the stator. The actuator provides a significantly lower out-of-plane force for a given in-plane force. The actuator will provide an in-plane force in the range of forces required in an advanced memory device when driven with actuation voltages in the range that can be controlled using CMOS integrated circuits. The actuator can be manufactured using micromachining techniques that employ processing similar to that used to make integrated circuits.
The electrostatic actuator includes a stator having a first linear array of electrodes disposed along an opposed surface and a rotor having a second linear array of electrodes disposed along an opposed surface opposite the opposed surface of the stator. The opposed surfaces of the stator and rotor are spaced apart by a spacing d. The rotor is supported relative to the stator to allow the rotor to move in the in-plane direction, parallel to the opposed surfaces. Initially, an alternating voltage pattern is imposed on the electrodes on both the rotor and stator. For example, a first voltage level is applied to every other electrode in each array, and a second voltage level, different from the first voltage level, is applied to each electrode adjacent the electrodes at the first voltage level. By introducing a local disruption into the alternating voltage pattern on the stator, the rotor can be moved in the in-plane direction by a precise distance.
The alternating voltage patterns will not by themselves reduce the out-of-plane attractive force to a level comparable with the large in-plane force. To reduce the out-of-plane force for a given in-plane force, the pitch/spacing ratio p/d, which is the ratio between the electrode pitch p of the rotor and the spacing d between the opposed surfaces of the rotor and the stator must be within an optimal range. A usably low out-of-plane force results when the pitch/spacing ratio is less than eight. The out-of-plane force is minimized for a given in-plane force when the pitch/spacing ratio is less than about 2.25.
In a preferred embodiment, in-plane motion is provided by a stepper driven array of electrodes located on the opposed surface of the rotor and a corresponding stepper drive array of electrodes located on the opposed surface of the stator. Each driven array has an even number n.sub.r of rotor electrodes and each drive array has an odd number n.sub.s, of stator electrodes, so that n.sub.s =n.sub.r.+-.1. The ratio of the pitch of the driven electrodes to the pitch of the stator electrodes is n.sub.s /n.sub.r.
The drive electrodes may alternatively be located on the rotor, in which case, the driven electrodes are located on the stator. In this case, each driven array has an even number of stator electrodes and each drive array has an odd number of rotor electrodes, differing in number from the number of stator electrodes by one. The ratio of the pitch of the driven electrodes to the pitch of the stator electrodes is equal to the ratio of the number of rotor electrodes and the number of stator electrodes.
As described above, an alternating voltage pattern initially exists on the electrode arrays located on each of the stator and the rotor. The alternating voltage pattern on the stator alternates between the first voltage and the second voltage, where the first voltage is applied to the first electrode in the stator array. In-plane movement of the rotor is induced by locally disrupting the initial alternating voltage pattern by switching the voltage on the first electrode from the first voltage to the second voltage. Further in-plane movement may be induced by switching the second stator electrode to the first voltage, leaving the voltage on the first electrode unchanged at the second voltage.
The mode of operation just described subjects the rotor to an attractive force directed toward the stator. However, unlike the case in which all the rotor electrodes are held at the same voltage, the magnitude of the out-of-plane attractive force can be reduced by a factor of approximately ten to a level approximately equal to the maximum in-plane force applied to the rotor. If the rotor is suspended by beam flexures, this reduction in the out-of-plane force by a factor of ten reduces the aspect ratio of the beam flexures to one that can be easily manufactured by conventional techniques.
A further advantage of the electrostatic actuator according to this previous invention is that it provides a large in-plane force for a given actuation voltage. The in-plane force may be as large as one third of the attractive force between the two plates of an equivalently-sized parallel-plate capacitor.
The electrostatic actuator according to this previous invention has another advantage in that the in-plane position of the rotor can be progressively stepped without changing the alternating voltage pattern imposed on the rotor electrodes. As a result, the stepping rate is not limited by the dynamic electrical characteristics of the rotor. Moreover, only one stator electrode in each set of stator electrodes need be switched at any one time to step the rotor position. This imposes a minimum of timing constraints on the stator voltage control circuitry.
The electrostatic actuator according to this previous invention can be used to provide displacements in both the in-plane and out-of-plane directions, i.e., in directions respectively parallel and perpendicular to the opposed surfaces of the stator and rotor. Opposed electrodes on which an alternating voltage pattern is imposed can also be used to exert an out-of-plane force on the rotor. Such out-of-plane force can be used to offset the out-of-plane attractive force exerted on the rotor by the electrodes generating the in-plane force. Additionally or alternatively, the out-of-plane force can be used to control the position of the rotor in the out-of-plane direction.
The out-of-plane force is preferably provided by an array of levitator drive electrodes located on the opposed surface of the stator and an array of levitator driven electrodes located on the opposed surface of the rotor. The arrays of levitator drive electrodes and levitator driven electrodes have equal pitch. The alternating voltage patterns are imposed so that levitator electrodes in the higher-voltage state on the rotor are aligned with the levitator electrodes in the higher voltage state on the stator. With this arrangement, the rotor is repelled from the stator. The out-of-plane force can be controlled by varying the voltages on either or both of the rotor or stator. The array of levitator drive electrodes may alternatively be located on the opposed surface of the rotor and the array of levitator driven electrodes may alternatively be located on the opposed surface of the stator.
Electrode arrays primarily generating an in-plane force ("stepper electrode arrays") and electrode arrays generating an out-of-plane force ("levitator electrode arrays") can be combined to provide additional functionality. For example, a number of similar electrode arrays can be used to increase the force applied by a single electrode array. Moreover, a first stepper electrode array disposed perpendicular to a second stepper electrode array can move the rotor in either or both of two perpendicular in-plane directions. A levitator electrode array disposed perpendicular to a stepper electrode array can be used to offset the attractive out-of-plane force generated by the stepper electrode array. Finally, parallel stepper electrode arrays with different pitches can be used to exert an in-plane force on the rotor without any associated out-of-plane force.
The out-of-plane force exerted by a stepper array may also be reduced by filling the space between the rotor and stator with solid or fluid dielectrics.
One half of the electrodes in the rotor array may be replaced by a conductive plane set to a predetermined voltage, such as ground potential. This conductive plane forms "effective" electrodes between adjacent physical electrodes. For example, a conductive plane may be formed, and may be covered by an insulating layer on which a linear array of electrically-interconnected physical electrodes is located. Each region of the conductive plane between adjacent physical electrodes functions as an effective electrode. The alternating voltage pattern is established by setting the electrically-interconnected physical electrodes to a voltage different from that of the conductive plane.
The drive electrodes in the stepper array are preferably connected to the same pair of voltage levels as the corresponding driven electrodes, although additional positioning accuracy can be provided if the electrode whose voltage is changed to disrupt the alternating voltage patten is connected to a voltage intermediate between the pair of voltage levels.
Because of the high voltage-to-in-plane-force conversion efficiency of the electrostatic actuator according to this previous invention, the pair of voltages applied to the electrodes to impose the alternating voltage pattern may be selected to provide compatibility with CMOS circuits. Voltage pairs differing by less than 20 V will provide rapid movement of the rotor over a 50 .mu.m range.
Throughout this disclosure, the terms "rotor" and "translator" are used to describe the movable part of the actuator, irrespective of whether the moveable part actually moves, and irrespective of whether it moves laterally or rotates. The embodiments described below can be directly employed in both rotary and linear actuators. In rotary electrostatic actuators, the electrodes of the stepper arrays are deposed radially about the center of rotation, and the electrodes of the levitation arrays are concentric with the center of rotation.
Arrays of sense electrodes may additionally or alternatively be located on the opposed surfaces of both the rotor and the stator to generate electrical signals indicating the position of the rotor. Corresponding sense arrays on the rotor and the stator have equal pitch. An alternating voltage pattern is applied to the sensor drive array, which is preferably located on the rotor, the voltage pattern induced in the sensor driven array preferably located on the stator is detected, and the position of the rotor is determined from the voltage pattern.
As noted above, the driven role of the rotor may be interchanged with the driving role of the stator for the stepper and levitator electrodes, and the driving role of the rotor may be interchanged with the driven role of the stator for the sensor electrodes.
As mentioned above, a usably-low out-of-plane force is obtained when the pitch/spacing ratio, p/d, is less than eight, and the out-of-plane force for a given in-plane force is minimized when the pitch/spacing ratio is less than 2.25. When the pitch/spacing ratio is less than 2.25, the in-plane force is maximized and the attractive out-of-plane force is minimized for a given actuation voltage.
Because the alternating voltage pattern on the rotor does not need to change with time, the electrostatic actuator according to this previous invention will also operate when the alternating voltage pattern is established on the rotor opposed surface in some other way. For example, the alternating voltage pattern may be established by electrostatic charge deposited on the opposed surface, by a poled ferroelectric located on the opposed surface or by a strain field established in a piezoelectric material located on the opposed surface. To describe these alternatives, the relationship to maximize the in-plane force in terms of the spacing d can be stated in terms of the primary spatial wavelength .lambda. of the voltage distributions on the opposed surfaces of the rotor and the stator. This more analytic description is also necessary for an accurate description of the electrostatic actuator when the voltage pattern imposed on the electrodes is not exactly an alternating pattern, or when intermediate voltage levels are applied to some of the electrodes.
When the operation of the electrostatic actuator is described in terms of the primary spatial wavelength, the relationship to maximize the in-plane force in terms of the spacing d can be stated as a requirement that ratio of the primary spatial wavelength .lambda. to the spacing d, i.e., the spatial wavelength/spacing ratio, be less than 4.5. If the voltage pattern is strictly alternating and the pitch of the electrodes is uniform and equal top, the primary spatial wavelength is simply 2p, and the constraint on .lambda. is identical to the constraint on p. If the voltage pattern is not strictly alternating, such as occurs when the alternating voltage pattern on the stator is locally disrupted to change the position of the rotor, then the primary spatial wavelength .lambda. is determined by calculating a Fourier transform of the voltage distribution.
The primary advantage of the electrostatic actuator according to this previous invention is that a voltage compatible with CMOS circuits will generate an in-plane force sufficiently large to move the rotor relative to the stator over distances of several tens of microns, and that the out-of-plane force generated as a side effect of generating the in-plane force is small enough to allow conventionally-fabricated folded beam flexures to support the rotor. A second advantage is that only two voltages need to be connected to the rotor, which enables the rotor to be fabricated with a minimum number of electrical interconnects. Moreover, because the voltages on the rotor are static, these electrical interconnects can have a relatively high impedance. Simplified electrical interconnects reduce the process complexity and minimize the effects of residual mechanical strains resulting from the use of dissimilar materials.
Although the previous invention has advantage over the prior art in the areas of long travel, high force, and two-dimensional in-plane motion of a monolithic element suspended by flexures, it operates as a stepper motor. Moreover, there continues to be a need for increased in-plane stiffness for the same maximum in-plane force level, greater resolution for a given minimum lithographic feature size, and elimination of out-of-plane forces. Like the previous invention, such an electrostatic actuator and a way controlling such electrostatic actuator should provide precise positioning and be controlled using CMOS integrated circuits. Such an electrostatic actuator should be fabricated using micromachining techniques that employ processing similar to that used to make integrated circuits.